Study On The Growth,Evolution And Boron-Doping Modification Of Tic In Al Alloys

Titanium carbide (TiC) has attracted much attention because of its high melting point, high modulus, great hardness, high chemical stability, etc. It is widely used as the reinforcing phase in Al, Ni, Fe, and Cu matrix composites, as the coating material on cutting tools and as the heterogeneous nucleus of a-Al in grain refinement of aluminum alloys. For these various applications, the crystallization shape, the surface roughness and exposed facets of TiC particles have important influence on their physical and chemical properties, especially for the catalytic application and surface heterogeneous nucleation ability. However, it exhibits non-stoichiometry over a wide range of C/Ti atom ratios (denoted as x) from x=0.47to about0.98, which makes the structural instability in the Al melt. It is found that TiC will react with Al in the melt and Al4C3is formed at the interface, which not only make the grain refining efficiency of Al-Ti-C master alloy fade seriously, but also lower the mechanical properties of the composites. Therfore, it is important to improve the stability of TiC in Al melt for both high efficiency master alloys and aluminum matrix composites. In this paper, the growth and evolution mechanism of TiC in Al alloy were systematically studied. Based on the influence of C vacancies in TiC on the structural stability and nucleating ability, B doping modification was proposed and a novel high efficiency Al-Ti-C-B master alloy was invented. Furthermore, the transformation mechanism of TiC in Al melt was also studied and applied to develop new materials.The main results can be describes as follows:(1) Crystal growth of TiC and its morphology evolution mechanism in Al alloyThe growth mechanism and final morphology of TiC crystals are determined by the intrinsic crystal structure of TiC and specific external growth conditions in the melt. TiC is a typical faceted crystal with a NaCl-type structure and trys to form the equilibrium shape of perfect octahedron with minimized total surface free energy. For TiC in the Al-Ti-C system,{110} faces with highest surface energy and highest growth rates disappear gradually and degrade to the eight edges. With the further growth,{100} faces shrink gradually and degrade to six corners, whereas{111} faces expand correspondingly, which leads to the formation of perfect TiC octahedron enclosed by eight complete{111} facets.However, the variations of external conditions usually change the growth rates along the<100> and<111> directions, which leads TiC to develop into other shapes, such as truncated octahedron, cuboctahedron and hopper.It is revealed that transition-metal atoms, such as Ni, Fe and Co, selectively absorb on{100} facets of growing TiC crystal due to interaction of Ni-3d and C-2p and reduce the interfacial energy of{100}. Due to the increase of relative growth rate along<111> direction,{100} faces on TiC crystal expand gradually with the areas of {111} surfaces decreasing, and then the shape of TiC crystal will turn into a perfect cube enclosed by{100}.Oriented attachment is an important growth mechanism and involves spontaneous self-organization of growth units bonding on{111} planes to reduce the total surface energy. Because the faster growth rate of each unit on the eight edges than that of {100} facet, the symmetric hollows appear on the face centers of the cubic skeleton with coarse edges and shrink gradually with the growth of TiC crystal.(2) The influence of B doping on the morphology, structural stability and heterogenous nucleating ability of TiCDue to the existence of carbon vacancies in TiC, B solute atoms can dope into the crystal lattice of TiC during its growth process. It is found that the B doping effect can be reflected directly on the XRD patterns. When the B was added during the formation process of TiC, the (200) plane is obviously preferred oriented and its reflection peak is the strongest, while the reflection peaks of (111) and (220) almost disappear. Furthermore, trace B dopant dissolved into the TiC structure and promoted the transformation of the TiC from octahedron to hexagonal platelet at the initial growth stage, and then changed the final morphologies of TiC crystals.A kind of Al-Ti-C-B master alloy with doped TiCxBy particles dispersed in the Al matrix was invented. Improved structure stability of TiCxBy due to the C vacancies substituted by B atoms is the main reason for the improvement of grain refining efficiency and fading resistance. Compared to Al-Ti-C master alloy, Al-Ti-C-B master alloy behaves a much better grain refining performance. The average grain size of α-Al can be reduced to170μm from3500μm by the addition of0.2%the prepared Al-5Ti-0.3C-0.2B. Especially the improved grain refining efficiency does not fade obviously within60min.(3) The transformation mechanism of TiC induced by excess B in Al meltThe transformation of TiC in Al melt is promted by excess B and the mechanism is revealed. Based on the diffusion and dopant of B into the lattice structure of TiC, it is found that the octahedron decomposes gradually in an exfoliation manner, and then the thin slice acts as the nucleating substrates for TiB2. Furthermore, during the transformation process from TiC to TiB2, C atoms are confined at the interface and then a layer of amorphous carbon-shell formed on the surface. The TiB2@C particles display hexagonal platelet morphology, with a length of about800nm and a thickness of about200nm, and the carbon layers with a thickness of4nm are homogeneously coated on the surface.A kind of Al-Ti-C-B master alloy with dispersive TiB2@C and Al4C3particles in the microstructure was invented. It is found that TiB2@C particles can act as the direcrt nucleating substrates without excess Ti for a-Al during solidification. Meanwhile, the Al4C3particles can act as the heterogenous nucleating substrates of a-Mg for Mg-Al alloys. Therefore, the Al-Ti-C-B master alloy is a kind of grain refiners for both commercial pure aluminum and Mg-Al alloys.